UC 356×368×129 vs HE 340 B
UC 356×368×129 is an UC section from the UK Blue Book (SCI P363); HE 340 B is the closest equivalent in the European (Continental) range. UC 356×368×129 carries more bending: 2480 cm³ of plastic modulus against 2410 cm³, 2.9% more. UC 356×368×129 is the lighter of the two at 129 kg/m against 134 kg/m, a saving of 3.7% on steel weight. UC 356×368×129 is deeper, 355.6 mm against 340 mm, which matters where the floor zone is tight.
| Property | UC 356×368×129 UC | HE 340 B HE | Difference |
|---|---|---|---|
| Mass | 129 kg/m | 134 kg/m | -3.7% |
| Depth | 355.6 mm | 340 mm | +4.6% |
| Width | 368.6 mm | 300 mm | +22.9% |
| Web thickness | 10.4 mm | 12 mm | -13.3% |
| Flange thickness | 17.5 mm | 21.5 mm | -18.6% |
| Area | 164 cm² | 171 cm² | -4.1% |
| Second moment, major (Iy) | 40,200 cm⁴ | 36,700 cm⁴ | +9.5% |
| Plastic modulus, major (Wpl,y) | 2,480 cm³ | 2,410 cm³ | +2.9% |
| Elastic modulus, major (Wel,y) | 2,260 cm³ | 2,160 cm³ | +4.6% |
| Second moment, minor (Iz) | 14,600 cm⁴ | 9,690 cm⁴ | +50.7% |
| Plastic modulus, minor (Wpl,z) | 1,200 cm³ | 986 cm³ | +21.7% |
| Radius of gyration, minor (iz) | 9.43 cm | 7.53 cm | +25.2% |
| Torsion constant (It) | 153 cm⁴ | 263 cm⁴ | -41.8% |
UC 356×368×129 to BS EN 10365:2017; HE 340 B to EN 10365 · Continental Steel. Difference is expressed relative to HE 340 B.
How to choose between them
On stiffness, UC 356×368×129 has the larger second moment of area: 40,200 cm⁴ against 36,700 cm⁴, a difference of 9.5%. This matters more often than the strength comparison, because on normal floor spans deflection rather than bending capacity sets the beam size - so if the two are close on modulus but apart on inertia, the stiffer one is usually the better answer even when it is not the stronger one.
Measured as bending capacity per kilogram of steel, UC 356×368×129 is the more efficient section: 19.2 against 18.0 cm³ per kg/m, 6.9% better. Over a floor of repeated beams that difference is real tonnage, though it is often outweighed by whichever section the fabricator can actually source.
The two diverge much more about the minor axis: UC 356×368×129 has 1200 cm³ of minor-axis plastic modulus against 986 cm³, 21.7% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. UC sections are specified to BS EN 10365:2017 and HE to EN 10365 · Continental Steel; the steel grades, tolerances and available lengths differ, connection details and bolt gauges are set out differently, and the two are rarely both stocked in the same market. Check availability before changing a section on a drawing to save 3.7% weight.
In short, UC 356×368×129 is the stronger section and UC 356×368×129 is the lighter one; where those are the same section, it is the clear choice, and where they are not, decide on whether your design is governed by capacity or by tonnage.